Automotive interior trim articles such as instrument panels, door panels, armrests, headrests, floor consoles, knee bolsters, and glove compartment doors conventionally have been constructed by applying a soft decorative covering over a rigid substrate mountable in an automobile vehicular body, with a cellular polyurethane padding interposed between the decorative covering and rigid substrate. A predetermined texture and color is usually provided to the decorative covering in an effort to simulate the appearance and feel of authentic leather.
The preparation of a self-supporting synthetic automotive interior trim article having a cast polymeric covering, such as a poly(vinyl chloride) (“PVC”) covering is known in the art and disclosed in, for instance, U.S. Pat. Nos. 4,562,025, 4,621,995, 4,623,503, 4,923,657, and 5,484,273.
In accordance with these disclosures, slush-or rotational-cast PVC is produced by introducing a liquid plastisol or tumbling a dry PVC powder in a metal mold until the material is formed against a moderately heated mold surface in a gelled state. The gelled material is then fused by heating the material to its fusion temperature and thereafter cooling the material to provide the PVC-based covering in a thermoplastic solid state.
However, in the commercial environment it has been found that PVC coverings prepared in accordance with the conventional techniques set forth in these disclosures must include several chemical additives. These chemical additives are required for at least the following reasons: to ensure that the product has a cosmetically consistent appearance that ages uniformly in use; to protect the coverings from degradation due to excessive temperatures that the coverings encounter during manufacture and use; and to bind the ingredients of the covering in a sufficient manner so as to prevent non-uniform surface appearance due to molecular separation of volatile and migratory ingredients during fusion in manufacture and long term exposure at elevated temperatures, which are often experienced in normal use.
While these additives, which can include heat and ultraviolet light stabilizers and plasticizers, serve the above-mentioned benevolent functions, the additives also can deleteriously affect the appearance of the resultant covering. For example, during manufacture in the rotational casting process disclosed in these related art disclosures, it is not unusual for some of these chemical additives to build up and deposit on the tool surface. Such deposits can hinder the casted material from completely filling the interstices of the grained surface of the mold, thereby preventing the exterior surface of the covering from fully transcribing the grained configuration of the mold surface and causing localized gloss patches to appear on the covering surface. Uniform appearance of color and gloss during manufacture is, therefore, difficult to obtain on a consistent basis without frequent mold cleaning. However, the additional time required to periodically clean the mold surface after demolding of a covering is accompanied by a corresponding loss in productivity. Further, frequent cleaning of the mold surface can cause wear on the grained configuration and shorten the tooling life.
A need therefore exists to provide a process for making a panel structure containing a decorative covering with a casted layer, such as a PVC layer or thermoplastic polyurethane layer, in which the panel structure can be produced in a more efficient and cost effective manner, and in which the resulting panel structure has a high quality, uniform leather-like appearance, and improved grain definition and durability.